The present application relates to semiconductor devices containing permanent charges, and more particularly to edge termination structures and methods.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
Permanent charges can be implanted into device material, supplied, for instance, by the implantation of certain atomic species, such as cesium, into a compatible material, such as silicon dioxide.
Embedded permanent charges have been used in the design of devices such as depletion mode vertical DMOS transistors. In published U.S. applications 2008/0191307 A1 and 2008/0164516, which are both hereby incorporated by reference, permanent or fixed charges were used to fabricate efficient high voltage devices with low specific on-resistance. (These published applications have overlapping inventorship, ownership and pendency with the present application, and are not admitted to be prior art.) FIGS. 1(a), 1(b), and 2 are generally based on these applications.
FIGS. 1(a) and 1(b) show examples of high voltage devices utilizing positive permanent charge. Permanent charge is included in a trench 104 filled with dielectric material 108. The devices may be constructed using a p+ layer 102 on a more lightly doped p layer 120 (which may be an epitaxial layer) On top of an N+ or N/N+ bottom layer 130.
The example in FIG. 1(a) is a high voltage diode. Here the front-side p-type diffusion 102 serves as an anode, and the backside N+ 130 (which may be the entire substrate) serves as a cathode. Under reverse bias the depletion zone at the junction 120/130 will spread mainly into p type layer 120. If the doping and thickness of layer 120 are chosen correctly, the electric field does not get high enough to cause breakdown at the rated operating voltage. Front-side and backside metallization 106/107 provide external connections.
FIG. 1(b) shows a similar bulk structure, with additional components which form an n-channel MOS transistor. In this structure the deep n+ diffusion 130 operates as a drain, and the p-type diffusion 102 provides a shallow body diffusion (or body contact diffusion). However, some important elements are added: a shallow and heavily doped n-type source region 150 lies between the source contact diffusions 102 and the trenches 104. A deep body diffusion 103 is preferably deeper than the body contact diffusion 102. An insulated gate 140 is located within the trench 104, and selectively inverts the nearest portions of the deep body diffusion 102 to control conduction. (Under OFF conditions, a depletion region will spread from the body junction 120/130.
In both of these embodiments the dielectric filled trenches with embedded permanent charges help to control the spread of the potential under reverse bias, i.e. they help to reduce peak electric field. As discussed in the two applications cited above, the use of permanent charge included in the trenches also helps to improve on-state conductivity at a specified breakdown voltage,
Power semiconductor devices are often used in environments where transient voltages are inevitable. This can occur, for example, in motor drives, or other applications where inductance is present. A semiconductor material will “break down” (and become conductive) if the electric field becomes too high, and then typically remains conductive until the device is physically destroyed, or until the supplied current drops below a low holding current value. The area where breakdown occurs can therefore suddenly receive the entire energy stored in the system. In high-voltage devices of this kind, it is preferable that breakdown, when it occurs, should occur in the device array, since the device area has much larger area than the termination area.
A high-voltage power device structure must have some edge termination, to provide lateral space for the voltage drop between the source and drain terminals. (Conduction along the edge of the die is hard to prevent.) Thus it is highly desirable, especially with high-voltage devices, that the edge terminations should have a breakdown voltage similar to or greater than that of the device active area. FIG. 2 shows a simple termination structure 210 surrounding an active device area 220, where the active area might include diodes, JFETs, MOSFETs, IGBTs, or other device types. Such a structure might use p-on-n+ epitaxial material as a starting point. However, the presence of the thick P layer 120 in the edge termination area precludes the use of many conventional termination structures such as Field Plates, Guard Rings or Junction Termination Extension (JTE).